Friction devices, such as clutches and brakes, of an automatic transmission of a vehicle are selectively engageable and disengageable to change gear ratios or alter the speed of the vehicle. For example, to shift from one transmission gear ratio to another, one clutch is disengaged and another clutch is engaged.
Electro-hydraulic valves are often used in automatic transmissions to control engagement and disengagement of friction devices, including transmission clutches. To achieve an acceptable shift quality, i.e., smoothly disengaging the off-going clutch and smoothly engaging the on-coming clutch, a typical automatic transmission electro-hydraulic valve must output a desired clutch pressure.
Electro-hydraulic valves used in automatic transmission clutch trim systems are available in many types, including variable bleed solenoids and related devices. In general, all of these devices receive an electrical input from electrical circuitry, such as an electronic or electrical controller, and provide an amount of output pressure that is a function of the amount of electrical input. Normally, the electrical input is electrical current. The relationship between the output pressure and the electrical input is defined by a transfer function.
The solenoid transfer function often varies from valve to valve even among valves of the same type. Solenoid valve manufacturers are therefore often challenged to minimize valve-to-valve variations in the command-to-output transfer function. During manufacture, valves are typically adjusted at their “end of line” test to keep the transfer function characteristic curve within specified limits. Such adjustments shift or offset the characteristic curve along the electrical input axis but do not significantly alter the overall curve shape or slope.
While the offset adjustment is helpful in reducing valve-to-valve variations, valve rejects still exist and add to production costs. Even “good” valves still retain some detrimental part-to-part variation within their specified limits.
Additionally, existing solenoid calibration systems require individual solenoid characterization data to be captured during solenoid manufacture and then loaded into the on-board transmission controller during transmission manufacture. Such systems are not usable after transmission manufacture; for example, when individual solenoids may need to be replaced in a service environment.